The advent of printed circuit boards has made possible the efficient assembly of highly complex electronic instruments such as computers, communication equipment, testing equipment, video equipment, etc. Printed circuit boards are commonly planar, flexible or inflexible sheets of material having thin, conductive lines or paths of metal on their surface and in their interior onto which electronic components can be mechanically attached in order to electrically interconnect the electronic components into an active circuit. The further advent of high density, double sided or multi-layered printed circuit boards that are designed to interconnect very large numbers of very large scale integrated circuits, integrated circuits, transistors, resistors, capacitors and other electronic components, required the development of technology for plating metal through holes in the board to interconnect the sides or layers of metal in the high density board.
During the manufacture of such metal clad printed circuit boards and such complex double sided or multi-layered plated-through printed circuit boards, close control over the deposition of copper is required so that neither too much nor too little copper is plated on the board or in the plated-through holes and that the copper deposit has a bright shiny uniform appearance with physical properties of high ductility and tensile strength. Methods for plating copper and other metals on a printed circuit board substrate involve the electrodeposition of a metal in which an electrical plating circuit is formed that causes dissolved copper ions in a plating solution, or plating medium, to combine with electrons on a printed circuit board hole or surface and be reduced to base metal. It has been found that the rate of electrodeposition of a metal from solution is important in order to efficiently and repeatedly form a uniform continuous smooth layer of bright shiny metal having a controlled depth or thickness with the good physical properties. Organic additives, used in electroplating baths, effectively control the rate of metal deposition, maintaining control over the physical properties of the plated metal as well as appearance.
Plating baths commonly contain at least one of two different types of additive. Additive systems can contain up to 10 different organic compounds in an electroplating formulation. A first type of additive is commonly called a brightener. This family of chemical constituents are commonly low molecular weight monomeric compounds which must be maintained in a range of concentration in the plating bath of about 0.1 to 1,000 parts of additive per million parts of plating bath in order to obtain acceptable electrodeposition of the base metal on the printed circuit board. The concentration of brightener can fluctuate due to the electrochemical destruction of the additive and the inclusion of the additive in the copper plate. As the brightener concentration drops, the copper electroplate can become coarse-grained or burned and powdery. In the instance that the brightener concentration is too high, the copper plate can again show a burned deposit with brittle or nonuniform results.
Plating baths can also contain one or more of a polymeric additives which are commonly called a leveler or a carrier. Similarly, the polymeric additives can be electrochemically degraded, mechanically degraded, or thermally degraded, resulting in a distribution of a family of polymeric additive by-products which must be quantified to monitor additive quality.
Plating baths can also derive organic contaminants from a number of sources including degradation products of additives, residual contaminants from other processing steps and the organic material from which the circuit board is made.
Individual contaminants that are derived from other processing steps are commonly virtually any type of organic compound that is carried forward from processing steps upstream of the copper plating bath which become physically or chemically associated with the printed circuit board substrate material. Upstream bath compositions function to clean, condition, etch, activate, accelerate or form the printed circuit board material before copper plating. The baths can contain anionic, nonionic, cationic and ampoteric surfactants, polymeric compositions, organic acids and bases, amines, hydroxy compounds, and many others. These compounds, while functional in the baths upstream of the plating bath, if carried forward into the plating bath can cause substantial reduction in plating efficiency, can alter the smoothness of the resulting copper plate, can reduce the purity of the resulting copper plate, can change the metallurgy of the resulting copper plate, and many other problems. Not only can the compounds which are intentionally placed in the baths upstream of the plating bath cause problems, but contaminants of the upstream baths can also be carried forward into the plating bath, causing similar problems.
Control of the copper plating process can clearly be enhanced by a detailed knowledge of the contents of each upstream bath and a clear detailed knowledge of the effect of the carry forward of bath additives and contaminants into the plating bath itself.
In the past, the quality of the plating medium has been tested using a Hull cell which is a small electrodeposition cell through which a current is passed and the nature of the copper electroplate is observed as it is plated in the cell. The concentration, purity, quantity of degradation or other quality of the additives and bath constituents can be roughly evaluated by viewing the nature of the electrodeposition. However, the Hull cell does not provide a direct measurement of additive concentration. Similarly, Tench, U.S. Pat. No. 4,132,605, teaches an electrical voltametric method of monitoring the concentration of additives by measuring the amount of copper plated or removed during a voltametric cycle. In this test the nature and amount of copper plated in the electrical test cell is observed but a direct measurement of the concentration of specified organic additive is not done. I am also aware that Zatco and others have attempted an analysis of organic components of copper plating baths using high pressure liquid chromatography techniques (70th AES Conference, June 1983). Our attempts to perform a reproducible quantitative routine analysis of the organic additives using the Zatco-type methods have not succeeded. We believe that the extremely corrosive, acidic or basic nature of plating baths, that we have attempted to analyze, interfere substantially with the chromatography columns used in the HPLC apparatus.
In liquid chromatography, more particularly high pressure liquid chromatography (HPLC), the sample to be analyzed is placed on the end of a column that commonly contains a silica support which is chemically or physically associated with a stationary phase. A solvent or mobile phase is directed through a chromatographic column and carries the sample over the stationary phase. The differences in affinity between the components of the analyzed material and the stationery phase on the support causes the components in the sample to separate, or be resolved, as the solvent carries the material through the column. The most common column support material is a silica composition which has the drawback that it can interact with both acidic or basic components of the plating bath, resulting in substantial interference with the separation of free components in the chromatographic column. We have attempted a prechromatographic neutralization of the acidic and basic components of the plating bath with a variety of techniques, but have found that the formation and removal of the resulting neutralization products or salts can cause substantial loss of organic material and the neutralization products can cause an unwanted interaction with the silica support during chromatographic analysis.
Accordingly, a substantial need exists for a reproducible, precise, accurate, routine HPLC method which suffers no interference from the highly corrosive acidic or basic nature of the bath, for the analysis of individual and collective organic constituents in electroplating baths and baths upstream of the plating baths that can be rapidly and routinely performed on a day to day basis to determine the types and state of the organic additives in a plating bath. Further such an HPLC method is essential if size exclusion chromatography, total organic carbon analysis and HPLC are to be successfully used to accurately reflect the total state of the plating bath organic constituent. These types of chromatographic techniques lend themselves very well to individual component control in terms of a feedback system for automated addition to allow consistent bath performance.